Sensors comprising micro-electrode arrays are used for measuring electrical activity in small networks of neurons. These sensors are often relatively big and only small matrices or arrays of micro-electrodes can be made. Current state-of-the art micro-electrode arrays (MEAs) may contain a maximum of 64 electrodes with a minimal spacing of 100 μm between neighbouring electrodes. The electrodes are often made of flat TiN pads with a maximum diameter of 10 μm. For some applications, smaller spacings, e.g., <10 μm, and a larger number of electrodes, e.g., >60000, may be required.
On-chip single cell recording of electrical activity using field-effect transistors has been demonstrated for large neurons or tissue slices. See P. Bergveld et al., IEEE Transactions on Biomedical Engineering, 1976; P. Fromherz et al., Science, May 1991; A. Cohen et al, Biosensors and Electronics, January 2004). In the case of mammalian neurons, e.g., hippocampal neurons, the cells are much smaller, which leads to a less efficient electrical coupling. In that case, the cells need to be tightly attached onto the chip surface and make a reliable electrical contact between the cell membrane and the recording device.
Various microneedles have been proposed, but whose size limits their functionality for use in in vitro experiments on small cells. See, e.g., Y.-G. Lv, J. Liu et al., Journal of Micromechanics and Microengineering, Vol. 16, No. 11, pp. 2492-2501 (2006); S. Khumpuang et al., Microsystem Technologies—Micro and Nanosystems—Information Storage and Processing Systems, Vol. 13, No. 3-4, pp. 209-214 (2007). Further, in EP 0803702, a microneedle apparatus is described in which a probe arm with a microneedle is cantilevered over an electronic circuit. Nevertheless, fixing of the probe arm on the electronic circuit is not straightforward because assembly of micro-fabricated parts is not easy.